Add Vega to D3.js section.

description of Vega from the website: "Vega is a visualization grammar, a declarative format for creating, saving and sharing visualization designs.
With Vega you can describe data visualizations in a JSON format, and generate interactive views using either HTML5 Canvas or SVG."
Fix URL for Vega (D3.js based library)
Merge pull request #171 from hangtwenty/master

Add Vega to D3.js section.